NMR techniques have been applied in a variety of environments, for such purposes as well logging, flow measurement and the monitoring of intra-tissue conditions.
It is well known that when a nuclear magnetic substance, such as water, is placed in a homogeneous static field (with a magnitude H.sub.0) its resonance angular frequency W.sub.0 is given by the equation:
W.sub.0 =GH.sub.0, where G is the nuclear gyro magnetic ratio of a measuring substance and is a natural constant.
Application of this phenomenon to the study of homogeneous or relatively homogeneous materials is well documented in the literature. On the other hand, application of NMR technique to inhomogeneous materials, for the selective study of regions of homogeneity within the inhomogeneous material has led to the necessity of using special techniques. For example, if appropriate magnetic fields are applied to an inhomogeneous body, the resulting nuclear magnetic resonance may include contributions from the various nuclear magnetic materials within the body subjected to the measurement. This can result in masking desired signals from one region by undesired signals from other regions.
Typically, in NMR spectroscopy, material being studied is subjected to both a static magnetic field and a radio frequency field, and the result is the induction of nuclear magnetic resonance when the above-stated equation is satisfied. Thus, a particular nuclear magnetic resonance indicates the presence of selected nuclei in the sample. Typically, the static magnetic field is produced by a suitable coil carrying a steady current, and in view of the magnitude of the magnetic field required, the coil may well be super conducting coil, the radio frequency field is produced by a supplementary coil or high frequency coil, supplied with high frequency current. Resonance is detected by a further or receiver coil surrounding the sample or the supplementary coil can be time shared.
Where the sample includes complex molecules, the localized fields produced by molecular electrons have a screening effect which causes identical nuclei in different chemical or molecular environments to resonate at slightly different frequencies. This effect, known as chemical shift, is typically of the order of 10.sup.-4 to 10.sup.-6 parts of the magnetic field. Provided that the magnetic field is sufficiently constant in space and time, these shifts, although small, can be detected and measured by the use of high resolution equipment and can yield valuable information about the chemical structure of a sample.
Where, as has been mentioned above, the sample or body being studied is inhomogeneous, and it is desired to subject a particular region in the inhomogeneous body to study, some further techniques are required to attempt to isolate the nuclear magnetic resonance signal derived from the volume of the body desired, from other portions of the body which may include other nuclear magnetic materials which can mask the desired signals. The known techniques, however, include scanning or mapping in which additional coils are provided to superimpose a sequence of switched magnetic field gradients or else time dependent magnetic fields onto the static field. Damadian, for example, in U.K. patent application No. 2,039,055A discloses a CW NMR spectrometer which requires scanning in frequency or magnetic field amplitude. Inevitably, the provision of additional fields and the analysis of the resulting signals requires elaborate arrangements of considerable complexity. In addition, the presence of gradients or time varying fields causes loss of information relating to chemical shift since the very gradients themselves destroy the homogeneity necessary to a resolution of this chemical shift information.